1. Field of the Invention
The invention relates to a method for correcting optical proximity effects (Optical Proximity Correction, OPC) when imaging a pattern, arranged in the object surface of a projection objective, into the image surface of the projection objective. The invention further relates to an optical filter, a method for producing an optical filter, and to a projection objective that is assigned at least one optical filter.
2. Description of Related Art
Projection objectives for microlithography are used in projection exposure machines for fabricating semiconductor components and other finely structured devices. These optical imaging systems serve for imaging patterns of photomasks or lined plates, which are arranged in the object surface of the projection objective and are generally referred to as masks or reticles, onto an article, arranged in the image surface of the projection objective and coated with a light-sensitive layer, with very high resolution on a demagnifying scale. When fabricating semiconductor components, the pattern is formed as a rule by lines and other structural units of a specific layer of the semiconductor component to be produced. The structures to be produced for the semiconductor components can include tiny metallic tracks and silicon tracks as well as other structural elements whose critical dimensions (CDs) can be substantially smaller than the wavelength of the ultraviolet light used for the imaging, and can, for example, be of orders of magnitude of 100 nm or thereunder.
Specific problems arise from the small size of the structures to be imaged, and from the finite size and other limiting factors of the projection system. For example, the high-frequency components of the light used for the imaging which are required for the imaging of sharp corners are partly incapable of passing through the projection objective, and therefore cannot contribute to the imaging. Moreover, scattered light which is produced at a structural unit is capable to a certain extent of influencing the imaging of a closely neighbouring structure, and this leads to a complex interaction of the electric fields for closely neighbouring structures. These effects are referred to as “Optical Proximity Effects”. In the case of the structures produced, these effects can cause the structures produced to deviate in a typical way from the associated shapes of the structures on the mask. These effects include, in particular, undesired line width variations as a function of the spatial density of neighbouring lines, as a result of which the signal processing speed of the semiconductor component can be impaired, and a shortening of the line ends (line-end shortening), it therefore being possible to lose a desired contact between abutting structural elements.
One contribution to diminishing these problems is the correction of optical proximity effects (Optical Proximity Correction, OPC). “Optical Proximity Correction” usually refers to a method with the aid of which the shapes of the structural elements on the mask are modified in targeted fashion in order to compensate the described nonideal properties of the lithography process. In this case, a desired shape is prescribed for the structural elements to be attained on the article to be structured, and the corresponding structural elements on the mask are modified in order to improve the reproduction of the critical geometries. In order to compensate the shortened line ends (line-end shortening), the corresponding line end of the mask structure is widened into a hammerhead shape. In order to compensate the rounding of corners, serifs are added to or subtracted from the corresponding corners of the mask structure in order to produce in the structured article corners whose configuration is closer to the desired, ideal corner shape. The task of conventional “Optical Proximity Correction” is therefore essentially to find in the case of a prescribed, desired geometrical structure of the article to be structured a mask design which leads to an actual structure which is as close as possible to the desired ideal structure while taking account of all the influences of the lithography process which vary structures.
Owing to the fact that the process windows in the microlithography fabrication of finely structured devices are becoming ever smaller, it is becoming increasingly more difficult and expensive to calculate and to fabricate mask structures which ensure a desired structure on the structured article. Consequently, the conventional methods of optical proximity correction are also becoming ever more complicated. In addition, small modifications to the process parameters of a lithography process can have the effect that the mask for a desired structure must be changed although the geometry of the desired structure does not change. This causes the lithography process to become more expensive and less stable overall.
U.S. Pat. No. 5,701,014 describes a method and a device for projection lithography in which the contrast produced in a radiation-sensitive coating material by optical proximity effects is removed in a single projection exposure step. A filter with two openings is inserted for this purpose in the region of a field plane of the projection system. One of the openings serves as opening for the passage of the radiation which is desired for the imaging and produces the desired structure in the radiation-sensitive material. A fraction of an inverse radiation passes through the other opening onto the radiation-sensitive material and thereby removes the contrast produced by optical proximity effects.